An estimated 145,290 new cases and more than 54,290 deaths attributed to colorectal cancer are expected in the United States in the year 2005. Approximately 20% of patients present with metastatic colon cancer at initial diagnosis, and another 25% ultimately develop distant metastases. Although advances in combination chemotherapy have improved outcome, median overall survival of patients with unresectable metastatic colorectal cancer following chemotherapy (5-fluorouracil (5-FU)/leucovorin with either irinotecan or oxaliplatin) is approximately 2 years. The vast majority of colorectal cancers harbor genetic alterations [loss/inactivation of p53;increased expression/activation of the epidermal growth factor receptor (EGFR)] that interfere with death signaling in response to DNA damage and limit the antitumor efficacy of chemotherapeutic agents. Our studies indicate that chemotherapeutic agents (irinotecan, oxaliplatin) induce p53-independent expression of ligands for NKG2D, a stimulatory receptor used by natural killer (NK) cells, ?d-TCR+ T cells and a[unreadable]-TCR+ T cells for immune surveillance of tumors. We find that these agents also cooperate with Apo2 Ligand/ Tumor necrosis factor- related apoptosis-inducing ligand (Apo2L/TRAIL), a key effector of NK cell- and T cell-mediated immunologic cytotoxicity, to induce p53-independent apoptosis of colon cancer cells. Specific Aim 1 seeks to establish a role of the immune system in the in vivo antitumor efficacy of chemotherapeutic agents in the treatment of colorectal cancers. The proposed studies will test the hypothesis that DNA damage-induced upregulation of NKG2D ligands enhances the sensitivity of tumor cells to the immune system and contributes to the in vivo antitumor efficacy of chemotherapeutic agents, and examine whether the cumulative depletion of natural killer (NK) cells or other immune effectors during the course of multi-cycle chemotherapy impairs the antitumor efficacy of chemotherapeutic agents. Specific Aim 2 will determine whether the efficacy of chemotherapy can be enhanced via strategies that exploit dendritic cell (DC)-NK cell interactions to activate the immune system [synthetic CpG oligodeoxynucleotides (CpG ODN) or infection with attenuated Listeria Monocytogenes (LM)]. Our premise is that DC-NK cell crosstalk may not only harness the innate immune system to improve the efficacy of chemotherapy, but also facilitate the generation of long-lasting adaptive immune responses to tumor antigens that are generated in the process of tumor killing. Although CpG ODN or attenuated LM can elicit antitumor immune responses, our studies indicate that colon cancer cells enhance their ability to withstand an attack by cytotoxic immune effector cells via EGFR-induced survival proteins that interfere with mitochondrial death signaling. Specific Aim 3 will test the hypothesis that a monoclonal antibody targeting EGFR (cetuximab) can enhance the antitumor efficacy and tumor-specificity of CpG ODN or attenuated LM by focusing systemically generated immune responses to EGFR+ tumor cells and sensitizing them to immunologic cytotoxicity. The proposed research seeks to uncover an instrumental role of the immune system in determining responses to chemotherapy and determine whether targeted immunotherapy with either CpG ODN or attenuated LM in combination with cetuximab can be used to improve the response of colorectal cancers to chemotherapy.